1. Field of the Invention
The present invention generally relates to a duplexer using surface acoustic wave filters.
2. Description of the Related Art
Recently, cellular phones and portable information equipment have spread widely along with the development of mobile communications systems. It has been considerable activity in downsizing and improving the terminal equipment among many manufacturers. Some cellular phones deal with both the analog and digital systems and employ a variety of frequency bands such as the 800 MHz–1 GHz band and 1.5 GHz–2.0 GHz band.
The recent development of cellular phones is directed to expanding the terminal functions that follow diversified system specifications. For instance, there are dual-mode cellular phones having the analog and digital systems, and dual-frequency cellular phones conforming to TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access). There are also dual-band cellular phones that handle two bands such as the combination of the 800 MHz band and the 1.9 GHz band, or the combination of the 900 MHz band and the 1.8 GHz or 1.5 GHz band. This trend requires improved performance of parts employed in these cellular phones, such as filters. It is also required to downsize the equipment and reduce the cost.
There are some types of antenna duplexers used in sophisticated terminal equipment. A dielectric type duplexer uses a dielectric for transmit and receive filters. A composite filter uses a dielectric for one to the transmit and receive filters and a surface acoustic wave (SAW) filter for the other. There is yet another type of duplexer that employs SAW filters only. The dielectric type duplexer has a relatively large size and has difficulty in downsizing and thinning the portable terminal equipment. The composite filter has the same problem as mentioned above.
The duplexer with SAW filters has a module type in which filters and a phase matching circuit are mounted on a printed-circuit board. An integral type has a ceramic multilayer package that houses the transmit and receive filters and the phase matching circuit. These filters have a volume approximately equal to ⅓ to 1/15 of that of the dielectric type duplexer and a height approximately equal to ½ to ⅓ of that. The downsized and thinned SAW duplexer can be produced at almost the same cost as the cost of producing the dielectric-type duplexer.
Further miniaturization of the SAW duplexer may be achieved by the use of a ceramic multilayer package as described in Japanese Patent Application Publication No. 10-126213, or by using a single chip on which two filters are mounted or employing the flip-chip mounting technique, which does not need bonding wires. Even when any of the means directed to further miniaturization, it is essential to employ a hermetically sealed package for housing the two filters and the phase matching circuit.
The facedown bonding contributes to downsizing. In contrast, the wire bonding restrains downsizing because it needs a large size particularly in height in order to arrange pads for wire bonding and prevent the wires from contacting a cap for hermetical seal. SAW duplexers that employ facedown bonding are proposed in Japanese Patent Application Publication No. 11-26623 or No. 2003-101385. However, these proposals are silent in how to reduce the influence of one filter characteristic to the other filter characteristic close thereto. It is desired that only a small influence as low as −40 dB to −50 dB be observed in the other filter.
Japanese Patent Application Publication No. 8-18393 discloses a line pattern sandwiched between ground layers for phase matching, the line pattern running on two layers of the package. The characteristic impedance of the strip line is designed to have a value greater than the characteristic impedance of an external circuit. However, this arrangement is required to have the distance between the line pattern and the ground patterns as long as possible in order to keep the characteristic impedance of the strip line higher than that of the external circuit. This requirement restrains downsizing. The ground layers that sandwich the line pattern are provided above footpads attached to the bottom of the package, so that there is difficulty in downsizing.